Protective immunity to infection by Eimeria parasites has been demonstrated to be dependent on T-cell mediated immune responses and may be associated with the release of cytokines. We have previously shown that the proportion of CD8-expressing T-cells in the peripheral blood of chicken increases transiently at 8 days after a primary infection with Eimeria tenella oocysts. The increase in the CD8+ population coincided with an increased proliferative lymphocyte response upon stimulation with E. tenella sporozoite antigen in vitro. In this study, we further investigated the functional activity of these peripheral blood leucocytes (PBL) by determining both the potential to proliferative and to produce IFN upon stimulation with E. tenella sporozoite antigens and mitogens. Enhanced proliferative responses to parasite antigen were accompanied by reduced responses to T-cell mitogens around 1 week of infection. The IFN activity in the supernatants of the stimulated PBL was measured by the ability to inhibit Semliki Forest Virus (SFV) replication in chicken embryo fibroblasts (CEF) and to activate macrophages, as measured by nitric oxide production. At eight days after infection the highest levels of virus inhibition and NO-production were detected upon stimulation with both E. tenella sporozoite antigen and mitogen. A strong correlation between the individual data of the two methods was found at this timepoint indicating that the produced cytokine was indeed IFN-gamma. These results suggest that around eight days after a primary E. tenella infection a parasite specific T-cell subset with the capacity of produce IFN(-gamma) is circulating which would be involved in the induction of protective immunity against Eimeria tenella.